Do you know why acid dyes are called that? Mostly acid dyes you will get are carboxylic or sulphuric acid salts. Often, they are applied from an acidic bath. That’s why they are called “acid dye”. Though acid dyes possess pretty good light fastness, the wet fastness is not that good. In this article, we are discussing Properties, Classification, Characters, and Mechanism of Acid Dye.
Acid dyes are mostly sulphuric or carboxylic acid salts and are essentially applied from an acidic bath and hence the name “acid dye”. Acid dyes have poor wash fastness, but light fastness is quite good. Dyes possess affinity for protein fibers and are mainly used to dye wool, silk, and nylon. Find Properties, Classification, Characters, and Mechanism of Acid Dye in the following:
Properties of Acid Dyes
These are the key Properties of Acid Dye:
- Acid dyes are anionic in nature
- Suitable for protein and polyamide fibers like silk, wool, nylon
- Acid dyes are applied from an acidic to a neutral bath
- Light fastness is quite good
- Overall, wash fastness is poor
- These dyes are not suitable for cellulosic fiber
- These dyes form a hydrogen, van der Waals, or ionic bond with fibers
Classification of Acid Dyes
Acid dyes are classified based on-
Acid dyes, based on their chemical nature
Acid dyes have a complex structure. They have sulphonyl or amino groups in their structure, thus making them soluble in water. Most of them belong to the following classes:
- Triphenylmethane type
- Azo type
- Anthraquinone type
Chemically, acid dyes are classified as:
- Monoazo, e.g., Metanil Yellow and Acid Orange 11
- Nitro e.g,. Picric acid
- Nitroso, e.g., Napthol Green B
- Triphenylmethane e.g,. Acid Magenta
- Xanthene, e.g., Rhodamine B
- Azine e.g,. Wool Fast Blue EB
- Quinolene Yellow PN
- Ketonimine e.g,. Sulfonine Grey G
- Antraquinone, e.g., Carbolan Green G
- Phtalocyanin,e e.g., Coomassie Turquoise Blue 3G
- Application or dyeing characteristics
Generally, acid dyes are classified based on their dyeing behavior. Dyeing behavior is observed especially according to the dyeing pH, wash fastness, and migration ability during dyeing. This dyeing behavior is detected according to the sulphonation degree of the dyeing molecule as well as the molecular weight of the dye particles.
Acid dyes based on strength
Acid dyes can be classified into four categories according to their strength. Here they are:
(1) Levelling, Strong or Equalizing acid dyes
These dyes can produce mostly levelled shades, and so they are known as levelling or Equalizing acid dyes. They possess poor fastness due to their low molecular weight with higher water solubility. These dyes are rapidly absorbed at a pH 2-,3 even at 40℃ and applied from a bath containing H2SO4 (3.5%, 168˚ Tw) and Glauber’s salt (10-20%) .
(2) Fast acid dyes
These dyes are usually mono-sulphonated acid dyes. They have a higher molecular weight than the typical levelling acid dyes. By using acetic acid (1–3% owf) and Glauber’s salt (5–10% owf), they can easily dye wool. Reasonable migration is seen at boiling temperature for these dyes.
Sometimes, a leveling agent is also required for the levelness of dyeing. Where level dyeing is necessary, these dyes can be used there. But at the time, when the perspiration and washing fastness of levelling dyes are inadequate, then these dyes can be used.
(3) Milling or Weak acid dyes
These dyes have better wash fastness than strong acid dyes. But they do not migrate well, and these are fast to the milling process imparted to woolen textile, thus called “Milling or weak acid dyes”. Wash fastness of these dyes is good due to the high molecular weight of the dye, rendering low solubility. Levelling property of these dyes is moderate because of affinity of the dye anion for the fiber is quite high.
(4) Super-milling or Neutral or Aggregated, or Very weak acid dyes
Dyes are applied from a neutral bath with acid-acid-liberating agent, e.g., Ammonium acetate, sulphate, or phosphate, which liberates acid beyond 80℃ to develop the required pH. Dye anion possesses a higher affinity for fiber at a minimum of acid. They are known as super-milling, aggregated, or very weak acid dyes.
Milling is a process where a woolen material is treated with considerable mechanical action in a mild alkaline solution to promote felting.
Characters of various types of Acid dyes are as follows
Dyeing of wool with acid dyes
Wool is a little bit sensitive to alkali. It shows moderate stability up to pH 10. But it shows much better stability in an acidic medium. In an acidic medium, it shows better stability up to pH 2. Its isoelectric point is pH 4.5 to 4.8. This pH is comparatively safer.
Let’s know the parameters for successful wool dyeing:
>Uniform pretreatment
>Levelness of dyeing
>Protection of wool
>Fastness property (Wet and light fastness)
>Anti-setting
>Rot – tip partition
>Spinnability
> Non-yellowing effect
Acid dyes have acidic groups in the structure, usually –SO3H. These acid groups react with basic groups present in the protein of wool or silk. Thus forming an organic salt by salt linkage. Wool exhibits amphoteric properties in solution. No matter acid or basic substances, it can absorb all of them. Depending on pH range, it can either be positive or negative. For wool, the isoelectric region is in the pH range of 4.7 to 7.2.
Typical recipe-
Wetting agent – 0.5 -1.0 g/L
Levelling agent – 1.0 – 3.0 g/L
Wool protecting agent – 2.0 – 4.0 g/L
Dyes – X%
Glauber salt – 10 – 20 g/L
Acetic/ sulphuric acid – 1 – 4 g/L (to maintain pH 4.5 – 5.5)
Temperature – 90 – 100°C
Time – 30 – 50 minutes
M:L – 1:10
Dye solution preparation
Add a little amount of water with dyes and make paste. Then add hot water to stir it. Thus the desirable dye solution is prepared.
Procedure –
- At 40 – 50°C temperature, prepare the dye bath with substrate.
- For maintaining 4.5 -5.5 pH, add levelling agent, acetic acid, salt and other auxiliaries.
- Run the bath for 10 – 20 minutes.
- Add dye solution at pH 4.5 – 5.5.
- Raise the temperature to 90-100°C over 20 – 30 minutes @ 1-2°/minute.
- Run the dye bath for 30 – 50 minutes for dyeing.
- Cool down the bath temperature to 60 – 70°C @ 2 – 3°C/minute
- Drop the dye bath and carry out the after-treatment process.
After-treatment process-
Successive hot and cold wash is given to remove the unfixed dyes from the dyed goods. It is done for improving colorfastness. Then it is treated with a suitable fixing agent.
Mechanism of Acid Dye
Under acidic condition, protein and polyamide fibers achieve cationic sites. As the amount of acid absorbed by the fiber increases with time, the more and more cationic sites are introduced under strong acidic condition. The dye anions then get attached with these cationic sites. This attachment between the dye anion and fiber takes place through hydrogen bond, van der waal’s force and ionic bond. The Mechanism of Acid Dye is an important thing to know.
In case of wool acid dye mechanism
Wool retains –NH2 and –COOH groups at either end of its structure which is able to make chemical reaction. Wool can be represented as: H2N- W- COOH where W retains the rest part of wool structure.
When wool is immersed in water, the H atom of carboxylic group of wool gets attached to the –NH2 group. Thus, wool chain acquire opposite electrical charges called zwitter ions:
At isoelectric point, it carries no net charge. As in this point there remains equal amount of positively charged ammonium groups and negatively charged carboxyl anions. For wool, the isoelectric region is between pH 4.8 to 7.
When acid is added to this bath, it releases hydrogen ions, which are taken up by the negative carboxylate ion of that zwitter ion containing wool and are transformed to an electrically neutral carboxylic acid group. Thus the positively charged ammonium groups are left available which act as dye sites for acid dye. At the same time, acid anions released by the acid are absorbed by the positively charged amino ends of the keratin macromolecules.
Amount of acid absorbed by the wool increases with acid treatment and generates more and more positive sites in wool. When the acid dye is added to the bath containing cationized wool, the dye anions get attached to the cation of wool through electrostatic force, with the liberation of salt.
Some of the group in the dyes are attached to the wool by hydrogen bonds and van der waal force. As dye anion is strongly held on the protein molecules than the acetate anion, the positive sites take up the acetate ions prior to addition of dye. As further dyeing procedure proceeds, ion transfer from wool to solution and solution to wool is continued.
This is the basic mechanism of acid dye in case of wool. Let’s know the criteria for better dye uptake for acid dyes.
Dye uptake depends on the nature and concentration of acid used. More the concentration of acid in bath, more the exhaustion of dye takes place. H3PO4 is recommended over H2SO4 for dyeing of wool. 1% H2SO4 = 2% H3PO4.
H3PO4 is suitable for wool because it causes less damage of wool as well as any blends of cellulosic fibers. It also produces better shades even when buffered with Na2CO3.
Acid dyes possess both the hydrophobic or non-polar and hydrophilic or polar characteristics. The hydrophobic part presents head and the hydrophilic part presents polar tail (-COOH, -SO3H).
Whereas, wool has a hydrocarbon or non-polar backbone of keratine. It feels higher attraction for the non-polar head of dye. This is the reason for higher affinity of acid dyes for wool; dyeing is thus analogous to extraction of non-polar solvent from an organic solvent.
The funny thing is, if you introduce a non-polar group in the dye structure such as –CH, it probably increases the affinity of wool for acid dyes. On the other hand, you can reduce the affinity of wool for acid dye with the introduction of polar group (-COOH, -SO3H) in the dye structure.
With the addition of electrostatic bond like NH3+D–, the dye anion is attached to wool with coordinate linkage. As a result, the strength is increased. As it converts from equalizing dyes to aggregated dyes. This causes improved fastness.
Influence of dyeing parameters
Electrolyte, acid, temperature all of these parameters have influence on the dyeing rate of acid dye. The influence of these parameters are discussed below:
Effect of Electrolyte
Electrolytes act mainly as retarding or levelling agent for strong acid dyes. For super milling dyes, which are applied from a neutral bath, salt plays an opposite effect. In this case, salt promotes dye uptake on protein fibers by reducing zeta potential as wool probably acquires a negative electrical potential in neutral solution.
Effect of Acid
Levelling and milling acid dyes are not taken up by protein fiber unless acid is added to bath. Addition of acid makes the fiber cationic in nature through attachment of H atom of acid to the COO– ion making it COOH. In the absence of acid in bath, marked by pH at or above 7.0 will cause repulsion of more dye anions with little dye uptake. The rate of exhaustion is dependent upon the acidity of the bath or pH of the bath.
Effect of Temperature
The reaction below is not possible at room temperature but only when the temperature of the bath is elevated, which helps in the acceleration of dye molecules. It helps to generate required momentum for the dye molecules. If dyeing begins at 40℃, optimum results can be achieved, then the temperature is raised slowly to a boil, and dyeing is further carried out at a boil for the desired time. Acid dyes are not transferred from the bath below 39℃.
To Wrap Up!
Acid dyes have wet and light fastness ranging from poor to excellent. Fastness property varies with chemical constitution as well as application class of dyes. Acid dyes exhaust well. No bleeding or fading occurs with washing. Of all the colorants, acid dyes produce the most luscious colors. Hope discussed well on the Properties, Classification, Characters, and Mechanism of Acid Dye.
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